1. Field of the Invention
This invention relates to apparatus for and method of measuring thermal transfer between an electrically powered sensor and a fluid.
2. Background Information
Measuring thermal transfer between a thermally active sensor and a fluid allows one to derive the fluid flow rate if the fluid's specific heat is known, or to derive the fluid's specific heat if its flow rate is known. A discussion of the use of thermal transfer measurements for correcting BTU meters is provided in the inventor's co-pending U.S. patent application Ser. No. 12/499,420, filed on Jul. 8, 2009, and now issued as U.S. Pat. No. 7,775,706. The disclosure of U.S. Pat. No. 7,775,706, is incorporated herein by reference.
Bialis, in U.S. Pat. No. 6,443,003, describes a sensor for measuring changes in mass air flow. His sensor uses one or more thermoelectric modules, each of which serves both as a heat transfer element and as a differential temperature sensor. That is, the thermoelectric module generates a temperature differential when powered and generates a Seebeck voltage when a temperature differential is present and the module is not powered.
In situations where the thermal transfer rates are low, the internal thermal resistance of the thermoelectric module can be ignored and the measured Seebeck voltage is representative of the temperature differential across its plates. The internal thermal resistance of the thermoelectric module is therefore of little concern when measuring low thermal transfer rates, but becomes significant when measuring high flow rates of fluids having a substantial specific heat.